Route finding in communications networks

ABSTRACT

A method of determining a restoration route (or an additional route) in a fully or partially meshed communications network of nodes, in which the step of sending a route-finder signature from a node to a neighboring node on one of a plurality of spare links of a span to the neighboring node includes the prior substeps of: determining whether the node has a higher or a lower ranking network node identity than that of the neighboring node; and if it is higher ranking, sending the route-finder signature on the spare link corresponding to the lowest ranking of the node ports associated with said span; or if it is lower ranking, sending the route-finder signature on the spare link corresponding to the highest ranking of the node ports associated with the span. Any contention which occurs because the two nodes connected to a span make provisional allocation for one or more links simultaneously to different restoration routes is dealt with by a contention protocol in which the highest ranking of the two nodes knows that its provisional allocation will be confirmed, and the lower ranking of the two nodes knows that it must send a backtrack signature for the capacity that is not available.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of finding, or determining, a routein a communication network; to a node arranged to perform the method;and to a network comprising such nodes. A route may be needed to replacean existing route which has failed, and such a route is referred to as arestoration route, or a route may be required to supplement an existingroute which is becoming congested. As used herein, the term "additionalroute" embraces both restoration routes and supplementary routes.

2. Description of Related Art

It is known, for example from the article "The Self-Healing Network: AFast Distributed Restoration Technique For Networks Using DigitalCross-Connect Machines", W. D. Grover, IEEE Globecom 87, and from U.S.Pat. No. 4,956,835 (Wayne D. Grover) to respond at the two nodes (knownas failure nodes) connected to a failed span to receipt of a spanfailure alarm to initiate a real-time restoration process.

The failure nodes determine on the basis of their unique networkidentities (IDs) which node acts as Sender and which node acts asChooser (also known as Master and Slave, respectively).

For each of the links of the failed span the Sender repeatedly transmits(floods) respective route-finder signatures to its neighbouring nodes(known as Tandem nodes) which forward flood the signatures to theirneighbouring nodes. In one embodiment in the abovementioned U.S. patenta node knows only its own identity (ID) and learns the ID of the node towhich connectivity has been lost by reading the last valid contents of areceive signature register on the affected port(s) corresponding to thefailed link(s), and in an alternative embodiment, a node stores andmaintains a neighbour node ID table.

The node which decides to act as Chooser now enters a waiting state andremains in it until it receives a route-finder signature. Then itresponds by transmitting a respective complementary reverse-linkingsignature (also known as a confirmation or return signature) to theTandem node from which the route-finder signature was received. Theconfirmation signature travels back through the Tandem nodesestablishing the required switch connections between node input andoutput ports, and eventually arrives at the Sender node, which thenceases transmitting the respective route-finder signatures, and proceedsto transmit on that newly established restoration route the trafficwhich would have been transmitted on the corresponding link of thefailed span.

The abovementioned U.S. patent also discloses that the restorationmechanism can be used for automatic provisioning of new circuit routesin a network by placing two nodes, between which it is desired toprovision additional (i.e. supplementary) circuit routes, directly intoSender and Chooser states with regard to an artificial fault between theselected nodes. The nodes would be supplied with artificial faultinformation including the number of circuit routings that are beingsought.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of determining an additional route in a fully or partly meshedcommunications network of nodes, the method comprising the step ofsending a route-finder signature from a node to a neighbouring node on aspare link of a span to the neighbouring node, and being characterisedby the prior steps of:

ranking the links of the span; and

determining on the basis of the respective unique network nodeidentities of the node and the neighbouring node whether the node is ina first or a second ranking relationship with respect to theneighbouring node;

and the method being characterised in that said sending step comprises:

if the node is in said first relationship, sending the route-findersignature to the neighbouring node on the lowest ranking of currentlyavailable spare links of said span; or

if the node is in said second relationship, sending the route-findersignature to the neighbouring node on the highest ranking of currentlyavailable spare links of said span.

An advantage of the present invention is that the two nodes at oppositeends of a span can independently allocate links from the set of sparelinks in the span for restoring failed routes, starting from the highestranked and lowest ranked spares, respectively, and thereby avoidcontention for the spares, or, at worst, limit contention to the onesituation in which both nodes simultaneously have provisionallyallocated the same spare or spares for two different restorations. Suchcontention is dealt with by a contention protocol in which, e.g. thehigher ranking of the two nodes knows that its provisional allocationwill be confirmed, and the lower ranking of the two nodes knows that itmust send a backtrack signature for the capacity that is not available.

Preferably, when said route-finder signature is a return route-findersignature, there are included the steps of:

detecting when one or more spare links of said span which have alreadybeen allocated by the node for a restoration route identified in a firstreturn route-finder signature sent to the neighbouring node arerequested for a restoration route identified in a second returnroute-finder signature subsequently received from the neighbouring node;and, in response,

if the node is in a predetermined one of said first and said secondrelationships, maintaining the allocation of said one or more sparelinks; or

if the node is in the other of said first and said second relationships,changing the allocation of said one or more spare links from therestoration route identified in said first return route-finder signatureto the restoration route identified in said second return route-findersignature, sending to the slave end node which originated said firstreturn route-finder signature a corresponding backtrack signature tocancel allocations for spare links corresponding to said one or morespare links, modifying the first return route-finder signature byreducing the content of a requested capacity field associated with therestoration route of said first return route-finder signature by thecapacity of said one or more spans, and sending said modified firstreturn route-finder signature to the neighbouring node.

Alternatively, when said route-finder signature is a return route-findersignature, there are included the steps of:

detecting that one or more spare links of said span which have alreadybeen allocated by the node for a restoration route identified in a firstreturn route-finder signature sent to the neighbouring node arerequested for a restoration route identified in a second returnroute-finder signature subsequently received from the neighbouring node;and, if the node is in a predetermined one of said first and said secondrelationships, and it is not an end node for the restoration routeidentified in said second return route-finder signature subsequentlyreceived from the neighbouring node; and, in response,

maintaining the allocation of said one or more spare links;

modifying the received second return route-finder signature by reducingthe content of a requested capacity field associated with therestoration route of said second return route-finder signature by thecapacity of said one or more spans; and

sending said modified second return route-finder signature to thecorresponding neighbouring node.

Preferably, said sending step comprises sending the return route-findersignature on each of the n lowest ranking, or highest ranking as thecase may be, of the currently available spare links of the span, where nis the content of a requested capacity field associated with therestoration route of the return route-finder signature.

According to a second aspect of the present invention, there is provideda node for use in a fully or partly meshed communications network ofnodes, the node being arranged to send, in use, a route-finder signatureto a neighbouring node on a spare link of a span to the neighbouringnode, and characterised by being arranged to determine, in use, on thebasis of the respective unique network node identities of the node andthe neighbouring node whether the node is in a first or a second rankingrelationship with respect to the neighbouring node; and

if in said first relationship, to send the route-finder signature on thespare link corresponding to the lowest ranking of the node portsassociated with said span; or

if in said second relationship, to send the route-finder signature onthe spare link corresponding to the highest ranking of the node portsassociated with said span.

Preferably, the node is further arranged to detect when, in use, one ormore spare links of said span which have been already allocated by thenode for a restoration route identified in a first return route-findersignature sent to the neighbouring node are requested for a restorationroute identified in a second return route-finder signature subsequentlyreceived from the neighbouring node; and, in response,

if the node is in a predetermined one of said first and said secondrelationships, to maintain the allocation of said one or more sparelinks; or

if the node is in the other of said first and said second relationships,to change the allocation of said one or more spare links from therestoration route identified in said first return route-finder signatureto the restoration route identified in said second return route-findersignature, to send to the slave end node which originated said secondreturn route-finder signature a corresponding backtrack signature tocancel allocations for spare links corresponding to said one or morespare links, and to send to the neighbouring node a modified said secondreturn route-finder signature in which the content of a requestedcapacity field associated with the restoration route of said secondreturn route-finder signature is reduced by the capacity of said one ormore spans.

Alternatively, the node is further arranged to determine when, in use,one or more spare links of said span which have been already allocatedby the node for a restoration route identified in a first returnroute-finder signature sent to the neighbouring node are requested for arestoration route identified in a second return route-finder signaturesubsequently received from the neighbouring node; and, in response,

if the node is in a predetermined one of said first and said secondrelationships, to maintain the allocation of said one or more sparelinks; and

if the node is not an end node for the restoration route identified insaid second return route-finder signature subsequently received from theneighbouring node, to send to the corresponding neighbouring node amodified said second return route-finder signature in which the contentof a requested capacity field associated with the restoration route ofsaid second return route-finder signature is reduced by the capacity ofsaid one or more spans.

Preferably, the node is arranged to send, in use, the returnroute-finder signature on each of the n lowest ranking, or highestranking as the case may be, of the currently available spare links ofthe span, where n is the content of a requested capacity fieldassociated with the restoration route of the return route-findersignature.

According to a third aspect of the present invention, there is provideda fully or partly meshed communications network of nodes, wherein thenodes are substantially identical and in accordance with the secondaspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A specific embodiment of the present invention will now be described byway of example with reference to the drawings in which:

FIG. 1 is a diagram of a network of interconnected nodes;

FIG. 2 is an enlarged portion of the network of FIG. 1; and

FIG. 3 is a diagram showing the connection of spare links between twonodes of the network.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The specific embodiment of the present invention relates to a real-timerestoration process for establishing a restoration route in acommunications network and the following description will be limited tothis although it will be appreciated that such a process need not be thesole restoration process in a network but can be combined with apre-planned restoration process.

In FIG. 1 there is shown a network 10 consisting of a number of nodes12, each having a unique network identity, linked by spans 14. For thepurposes of this description, node network identities are upper casereference letters, although in practice they are numerical, and onlynodes A to K will be referred to. To avoid possible confusion thereference letter "I" is not used. Individual spans 14 will be referredto by the two node identities of the respective pair of nodes betweenwhich a span is connected, e.g. span AB (or span BA, depending on, e.g.the direction of a signature).

Of the many routes in the network 10 between respective pairs of endnodes, this description will consider only three existing routes havingunique route IDs "X", "Y", and "Z". Route X is between end nodes A andE, passing through intermediate (also known as tandem) nodes B, C and D,and comprising a sequence of bidirectional links within spans AB, BC,CD, and DE; route Y is between end nodes A and G, passing throughintermediate nodes B, C and F and comprising a sequence of bidirectionallinks within spans AB, BC, CF, and FG; and route Z is between end nodesD and J, passing through intermediate nodes C, and H and comprising asequence of bidirectional links within spans DC, CH, and HJ. These nodesand spans are shown in FIG. 2 in which, for clarity, other nodes andspans are omitted.

The spans between the nodes comprise working links and spare links, andeach working link is part of a respective unique route. Route capacityis expressed in terms of numbers of circuits, but capacity is added, orsubtracted, in link units.

Within a span the links are sequentially ranked and identified by, inthis example, lower case reference letters. Specifically, with referenceto FIG. 3, span FK has ten working links a to j, (not shown) and sixspare links k to p. For convenience, a has the lowest ranking in thisdescription (but there is no reason why a should not have the highestranking). The links in a span are connected to node ports in aconnection numbering convention in which the lowest ranked link isconnected to the lowest ranked of corresponding ports at both associatednodes.

In this example, node F has eight spans to its neighbouring nodes (onlytwo being shown in FIG. 3), and the links a to p of span FK arerespectively connected to ports F101 to F116 of node F. Node K has fourneighbouring nodes and the links a to p are correspondingly connected toports K45 to K60 of node K.

Consider that an excavator has severed a duct close to node C andcontaining at that point both span CD and span CF. In this case, thenodes C and D, i.e. the failure nodes for route X, upon detecting thefailure of span CD, decide that a restoration route is to be foundbetween nodes A and E, and similarly nodes C and F decide that arestoration route is to be found between nodes A and G, and nodes D andC decide that a restoration route is to be found between nodes D and J.

For ease of explanation it will be assumed that no other routes havefailed, although it will be understood that, in practice, the failurenodes will, correspondingly, act to find a respective restoration routefor each of any other routes which have experienced a failed link. Theorder in which these restoration routes are established may bepredetermined by ranking the routes in priority order, but such rankingis not part of the present invention and will not be described.

Each failure node will, as the result of the abovementioned decision,now generate a common help signature (referred to herein as a helpmessage) for its associated failed routes, and send the common helpmessage to the respective end nodes of the routes.

Thus, node C will send its common help message for routes X, Y and Z tonodes B and H, because those are the neighbouring nodes for theseroutes, node F will send its common help message for route Y to node G,and node D will send its common help message for route X to node E. Itwill be appreciated that although node D is a failure node for route Zit is also an end node for that route.

Each of the various signatures used in the restoration process has aheader and a trailer, the header including a four bit signature typefield. The various types are normal common route-finder (also referredto as forward common route-finder), reverse common route-finder (alsoknown as backward common route-finder), route-tracer, alarm, commonhelp, backtrack, and return (also known as confirmation).

The information section of a common route-finder signature comprises afour bit flood count field, a four bit hop count field, a four bit routeID count field, one or more sixteen bit route ID fields, and acorresponding number of eight bit circuit number fields.

The information section of a common help signature comprises a four bitroute ID count field, one or more sixteen bit route ID fields, and acorresponding number of eight bit circuit number fields, so in the caseof the signature sent from node C to nodes B and H, its ID count fieldcontains the number three, the three ID fields contain, respectively, X,Y and Z, and the three associated circuit number fields contain therespective capacities of these routes.

As the common help messages pass through their respective intermediatenodes they break down connections in the corresponding failed route.Each node will forward a received common help message on the link orlinks which correspond to the route ID or IDs contained in the commonhelp message.

Each node knows its own network ID and contains a table storing theroute IDs for which it is an end node, and the network IDs of the otherend nodes.

When node B receives the common help message, it will check its storedtable to find out whether it is an end node for any of the identifiedroutes, and where, as in this case, it is not an end node for route X orfor route Y, it will transmit the common help message on the outgoinglinks associated with those routes (links of the span BA), and breakdown the connections for those routes by removing the route and linkdata from its connection table. Node H similarly forwards the commonhelp message to node J.

When node A receives the common help message, it will determine that itis an end node for the routes X and Y, and proceed to determine whetherit is higher ranking or lower ranking relative to the stored IDs of theother end nodes, i.e. E for route X and G for route Y, based on theunique network IDs (ordinal numbers) of the nodes. If the former, thenit will act as a master node (also known as a sender node), and if thelatter then it will act as a slave node (also known as a chooser node).In this example node A has a higher ranking network ID than both node Eand node G, and thus on receipt of the common help message will, forestablishing a restoration route for routes X and Y, assume the role ofmaster.

Similarly, when nodes E and G receive the respective common helpmessage, each will determine that it is an end node for a routeidentified in the help message, and proceed to determine whether it ishigher ranking or lower ranking relative to the stored ID of the otherend node for its associated route (X or Y). In this example both nodes Eand G have a lower ranking network ID than node A, and thus on receiptof the common help message, each will, for establishing a restorationroute for routes X and Y, assume the role of slave.

Node A, as master, now broadcasts a common forward route-findersignature for the failed routes X and Y, i.e. sends the signature onspare links to its neighbouring nodes. They in turn forward the receivedsignature, which thus floods through the network. This signaturecontains the IDs of the routes X and Y, the respective requestedcapacities for the routes, the number two in its route ID count field,and has its flood count field set to one. As the signature floodsthrough the network, the forwarding or relaying nodes (i.e. those nodeswhich are not end nodes for any route ID in the signature) increment thehop count field.

The relaying nodes forward the common signature on all spans. No checkis made to see whether the spare capacity on a span is sufficient forthe total capacity of a failed route, and the nodes do not mark thatcapacity as reserved.

The relaying nodes check the hop count of a received signature and takeno action if the count is greater than a predetermined value. This setsa limit to the geographical extent of flooding. In variants, floodingcontrol additionally or alternatively comprises checking a time oforigin field in the signature and taking no action if the signature isolder than a predetermined limit.

The master node A, when it has broadcast the common route-findersignature, will enter a quiescent state to await receipt of respectivereturn signatures.

Upon node E determining that it is to act as slave for the failed routeX, it starts (triggers) a timeout to await receipt of a correspondingroute-finder signature containing the route ID X and thus indicating apotential restoration route of unknown capacity. If no forwardroute-finder signature for route X has been received by the end of thetimeout period, node E will switch to act as master for route X and senda reverse route-finder signature to its neighbouring nodes, but thisaspect of restoration is not part of the present invention and will notbe described further.

On the first receipt of such a forward route-finder signature within thetimeout, the slave node E generates a return signature (also called aroute confirmation signature) and sends it back via the node from whichthe forward route-finder signature was received. This return signatureis similar to the route-finder signature, but differs in that thecontent of the signature type field is changed to identify the signatureas a return signature travelling towards the master node A, the route IDcount field is omitted, a single route ID field is used containing theroute ID X, and a single field is used for the requested capacity. Theslave node E ignores any subsequently received route-finder signaturesfor the route X.

In this example, it has been assumed that non-disjoint restorationroutes are permissible, and that the first forward route-findersignature for route X received by node E has traversed a route throughnodes B, C, K and F; the first forward route-finder signature for routeY received by node G has similarly traversed a route through nodes B, C,K and F, and that the first forward route-finder signature for route Zreceived by node J has traversed a route through nodes F, K and H. It isalso assumed that the requested capacity for routes X, Y and Z arethree, two and four, respectively.

As a return signature passes through the nodes of the potentialrestoration route, each of these nodes checks what capacity isavailable, makes appropriate connections between the correspondingswitch ports, and creates an eight bit node ID field, into which itwrites its node ID. The node compares the requested capacity with theavailable capacity, and if the requested capacity is not greater thanthe available capacity it will make connections for the requestedcapacity and send the return signature to the next node of the potentialrestoration route. However, if the requested capacity is greater thanthe available capacity, the node will make connections for the availablecapacity and forward the signature with the number in the requestedcapacity field replaced by the available capacity, and also send to theslave end node a backtrack signature containing the ID of the route andthe value of the difference between the requested capacity and theavailable capacity to take down connections that have already been madefor the capacity that cannot be established on that particularrestoration route.

Upon receipt of the return signature, the master node A knows that arestoration route now exists, as identified by the intermediate orrelaying node IDs in the signature, and knows the capacity of thatparticular restoration route, and now sends a route-tracer signature tonode E, via the restoration route, to inform it of the intermediatenodes of the restoration route. Where the invention is used to find asupplementary route, the route-tracer signature can be sent on theexisting route. This use of a route-tracer signature is known in the artand does not form part of the present invention.

In this specific example (FIG. 3), it is assumed that the returnsignature from node E (for route X) arrives at node F at substantiallythe same time as the return signature (for route Z) arrives at node Kfrom forwarding node H, and that E is higher ranking than F, and H ishigher ranking than K.

Node E will have sent the return signature on the lowest ranking sparelink of span EF, and this will have been received by node F on, say,port F15. Node F now does a number of things, namely,:

it determines the requested capacity for route X from the returnsignature (three) and allocates ports F15, F16 and F17 (three sparelinks of the span FE) for the restoration route, node E will havealready determined that span EF had sufficient spare capacity;

it determines the node to which it has to send the return signature forroute X and finds that this is node K;

it checks the span FK for the requested capacity and, upon finding thatthe requested capacity (three) is not greater than the spare capacity(six), now proceeds to determine the relative ranking of the nodeidentities F and K and, assuming that F is higher ranking than K,allocates the three lowest ranking links, k, l, and m, for therestoration route for route X, making connections between the threeports F15, F16 and F17 and three ports F111, F112 and F113 correspondingto links k, l, and m, on span FK;

it forwards the return signature to node K on spare link k, andsubsequently receives from node K a return signature for route Z.

Similarly, node K does a number of things, namely:

it determines the requested capacity (four) for route Z from the returnsignature forwarded by node H and allocates four ports, K7, K8, K9 andK10 on span KH, for the restoration route, node H will have alreadydetermined that span HK had sufficient spare capacity;

it determines the node to which it has to send the return signature forroute Z and finds that this is node F;

it checks the span KF for the requested capacity in the return signaturefor route Z and, upon finding that the requested capacity (four) is notgreater than the spare capacity (six), now proceeds to determine therelative ranking of the node identities K and F and, given that K islower ranking than F, allocates the four highest ranking links, p, o, nand m, for the restoration route for route Z, making respectiveconnections between the four ports K7, K8, K9 and K10 on span KH, andfour ports K60, K59, K58 and K57, respectively corresponding to links p,o, n and m, on span KF;

it forwards the return signature to node F on link p, and subsequentlyreceives from node F a return signature for route X.

Node K will receive on spare link k connected to its port K55 the returnsignature for route X requesting a capacity of three links, and willenter contention mode, since it has already allocated and connectedlinks p, o, n and m, and, similarly, node F will receive on spare link pconnected to its port F116 the return signature for route Z requesting acapacity of four links, and will enter contention mode, since it hasalready allocated links k, l and m. In this case, the contention will bein respect of which route will take precedence over the allocation ofspare link m.

On the basis of higher ranking node takes precedence, node K will:

allocate link m to the return signature for route X;

forward the return signature for route X with its capacity fieldunamended to node C;

change the link allocation for the restoration route for route Z tolinks p, o and n;

break down the connection between port K57 and the port K10 and make anew connection between port K57 and a port on the span KC;

generate a backtrack signature for route Z with the value one in thecapacity field to indicate the capacity deficit, and send the backtracksignature to node H via port K10.

Since the two nodes connected to a span know the port numbers of thespare links, it is sufficient for the return signature to be sent on thelowest ranking (or highest ranking, as the case may be) spare linkbecause the receiving node can determine, for a requested capacity of n,the n lowest ranked spare links. However, in variants, the forwardedsignature can be updated by the forwarding node to contain theidentities of the allocated links, or a node can send the returnsignature on all the spare links allocated for a route.

Suppose that node F now receives the return signature for route Y fromnode G, it will send a backtrack signature containing the deficitcapacity that it was not able to connect, in this case, two, and node Gwill respond by broadcasting a reverse route-finder signature for routeY.

As mentioned above, the node A will know from the content of the circuitnumber field in the received return signature for route X that thecapacity of the restoration route is the same as the requested value,and it will then send a route-tracer signature on the restoration routeA, B, C, K, F, E, to inform node E of the identities of the tandemnodes.

On the other hand, in the case of route Z, node D will know from thecontent of the circuit number field in the received return signature forroute Z that that the capacity of the restoration route (D, F, K, H, J)is less than the requested capacity, and will switch to act as a slavenode for route Z for the deficiency and await receipt from node J of areverse route-finder signature for route Z.

On receipt by node J of the backtrack signature sent by node K, theslave node J switches to act as a master node for route X, generates aroute-finder signature with its flood count field set to two, and withthe requested capacity in this signature set to the value in thebacktrack signature (i.e. the circuit shortfall), and sends it to itsneighbouring nodes. This signature is also referred to as a reverseroute-finder signature. It will be appreciated that signatures with oddflood counts can be identified as successive attempts made by theoriginal master to find a restoration route, and that signatures witheven flood counts can be correspondingly identified as successiveattempts made by the original slave.

The node A responds to first receipt of a reverse route-finder signaturesent by node G by switching to act as a slave node and immediatelysending a return signature on the link on which the reverse route-findersignature was received. This signature has the appropriate code for areturn signature in its signature type field, has its flood count fieldset to two, and also has its circuit number field set to the value inthe received reverse route-finder signature.

The node E, acting as a master and having sent out reverse route-findersignatures, will now be in a waiting state.

The abovedescribed method of finding a restoration route in a networkcan be used to find a supplementary route by sending instructions from anetwork control centre to the two end nodes of a congested route so thatthey treat the congested route as failed and initiate the method of theinvention to find an additional route (also known as an alternativeroute) between the two end nodes.

It will be appreciated that although the embodiment has been describedwith reference to a small network and that for convenience the number oflinks of a span has been correspondingly small (sixteen), in a largenetwork a span will comprise hundreds of working links and hundreds ofspare links.

An advantage of the abovedescribed embodiment is that the two nodes atopposite ends of a span can independently allocate links from the set ofspare links in the span for restoring failed routes, starting from thehighest and lowest ranked spares, respectively, and thereby avoidcontention for the spares until the limiting situation in which bothnodes simultaneously have provisionally allocated the same spare orspares for two different restorations. It will be appreciated that sucha limiting situation contention does not always occur, since theremaining spare or spares may be provisionally allocated by one node andconfirmed by the other node upon receipt of the return signature beforethat other node is ready to check the availability of spares and makeits provisional allocation for a restoration route. Any contention whichdoes occur is dealt with by a contention protocol in which the higherranking of the two nodes knows that its provisional allocation will beconfirmed, and the lower ranking of the two nodes knows that it mustsend a backtrack signature for the capacity that is not available.

We claim:
 1. A method of determining an additional route in a fully orpartly meshed communications network of nodes, the method comprising thestep of sending a route-finder signature from a node to a neighboringnode on a spare link of a span to the neighboring node, and includingthe prior steps of:ranking the links of the span; and determining on thebasis of the respective unique network node identities of the node andthe neighboring node whether the node is in a first or a second rankingrelationship with respect to the neighboring node;said sending stepcomprising: if the node is in said first relationship, sending theroute-finder signature to the neighboring node on the lowest ranking ofcurrently available spare links of said span; or if the node is in saidsecond relationship, sending the route-finder signature to theneighboring node on the highest ranking of currently available sparelinks of said span.
 2. A method as in claim 1, wherein said route-findersignature is a return route-finder signature, and also including thesteps of:detecting when one or more spare links of said span which havealready been allocated by the node for a restoration route identified ina first return route-finder signature sent to the neighboring node arerequested for a restoration route identified in a second returnroute-finder signature subsequently received from the neighboring node;and, in response, if the node is in a predetermined one of said firstand said second relationships, maintaining the allocation of said one ormore spare links; or if the node is in the other of said first and saidsecond relationships, changing the allocation of said one or more sparelinks from the restoration route identified in said first returnroute-finder signature to the restoration route identified in saidsecond return route-finder signature, sending to the slave end nodewhich originated said first return route-finder signature acorresponding backtrack signature to cancel allocations for spare linkscorresponding to said one or more spare links, modifying the firstreturn route-finder signature by reducing the content of a requestedcapacity field associated with the restoration route of said firstreturn route-finder signature by the capacity of said one or more spans,and sending said modified first return route-finder signature to theneighboring node.
 3. A method of determining an additional route in afully or partly meshed communications network of nodes, the methodcomprising the step of sending a route-finder signature from a node to aneighboring node on a spare link of a span to the neighboring node, andalso comprising the prior steps of:ranking the links of the span; anddetermining on the basis of the respective unique network nodeidentities of the node and the neighboring node whether the node is in afirst or a second ranking relationship with respect to the neighboringnode;said sending step comprising: if the node is in said firstrelationship, sending the route-finder signature to the neighboring nodeon the lowest ranking of currently available spare links of said span;or if the node is in said second relationship, sending the route-findersignature to the neighboring node on the highest ranking of currentlyavailable spare links of said span;wherein said route-finder signatureis a return route-finder signature, and also including the steps of:detecting that one or more spare links of said span which have alreadybeen allocated by the node for a restoration route identified in a firstreturn route-finder signature sent to the neighboring node are requestedfor a restoration route identified in a second return route-findersignature subsequently received from the neighboring node; and, if thenode is in a predetermined one of said first and said secondrelationships, and it is not an end node for the restoration routeidentified in said second return route-finder signature subsequentlyreceived from the neighboring node; and, in response, maintaining theallocation of said one or more spare links; modifying the receivedsecond return route-finder signature by reducing the content of arequested capacity field associated with the restoration route of saidsecond return route-finder signature by the capacity of said one or morespans; and sending said modified second return route-finder signature tothe corresponding neighboring node.
 4. A method as in claim 2 whereinsaid sending step comprises:sending the return route-finder signature oneach of the n lowest ranking, or highest ranking as the case may be, ofthe currently available spare links of the span, where n is the contentof a requested capacity field associated with the restoration route ofthe return route-finder signature.
 5. A node for use in a fully orpartly meshed communications network of nodes, the node including:meansto send, in use, a route-finder signature to a neighboring node on aspare link of a span to the neighboring node, and means to determine, inuse, on the basis of the respective unique network node identities ofthe node and the neighboring node whether the node is in a first or asecond ranking relationship with respect to the neighboring node; andifin said first relationship, to send the route-finder signature on thespare link corresponding to the lowest ranking of the node portsassociated with said span; or if in said second relationship, to sendthe route-finder signature on the spare link corresponding to thehighest ranking of the node ports associated with said span.
 6. A nodeas in claim 5, further including:means to detect when, in use, one ormore spare links of said span which have been already allocated by thenode for a restoration route identified in a first return route-findersignature sent to the neighboring node are requested for a restorationroute identified in a second return route-finder signature subsequentlyreceived from the neighboring node; and, in response, if the node is ina predetermined one of said first and said second relationships, tomaintain the allocation of said one or more spare links; or if the nodeis in the other of said first and said second relationships, to changethe allocation of said one or more spare links from the restorationroute identified in said first return route-finder signature to therestoration route identified in said second return route-findersignature, to send to the slave end node which originated said secondreturn route-finder signature a corresponding backtrack signature tocancel allocations for spare links corresponding to said one or morespare links, and to send to the neighboring node a modified said secondreturn route-finder signature in which the content of a requestedcapacity field associated with the restoration route of said secondreturn route-finder signature is reduced by the capacity of said one ormore spans.
 7. A node as in claim 5, further including:means todetermine when, in use, one or more spare links of said span which havebeen already allocated by the node for a restoration route identified ina first return route-finder signature sent to the neighboring node arerequested for a restoration route identified in a second returnroute-finder signature subsequently received from the neighboring node;and, in response,if the node is in a predetermined one of said first andsaid second relationships, to maintain the allocation of said one ormore spare links; and if the node is not an end node for the restorationroute identified in said second return route-finder signaturesubsequently received from the neighboring node, to send to thecorresponding neighboring node a modified said second returnroute-finder signature in which the content of a requested capacityfield associated with the restoration route of said second returnroute-finder signature is reduced by the capacity of said one or morespans.
 8. A node as in claim 6 and including:means to send, in use, thereturn route-finder signature on each of the n lowest ranking, orhighest ranking as the case may be, of the currently available sparelinks of the span, where n is the content of a requested capacity fieldassociated with the restoration route of the return route-findersignature.
 9. A fully or partly meshed communications network of nodes,wherein the nodes are substantially identical and as in claim 5.